This invention relates to material testing and defect removal in metal objects. In particular, this invention relates to an apparatus and method for removing relatively large metal sections from a metal object without producing stress concentrations at the removal site.
Large metal objects, such as turbine rotors for example, often operate in hostile environments. Turbine rotors are subject to large mechanical stresses resulting from their high speed rotation and large variations in operating temperatures. Over time, these stresses may compromise the mechanical integrity of the objects. Turbine rotor failure can be catastrophic, not only financially, but also in terms of the loss of human life. Therefore, preventative maintenance and repair is very important for turbine rotors as well as for numerous other large metal components used in industrial applications.
Technicians use metallurgy and standardized stress testing on metal samples taken from an object to determine the relative condition of the metal comprising the object. To achieve useful results with the tests, however, the samples must be of a minimum size and must be representative of the metal object as a whole. Therefore, the selection of a sampling site and the sampling process itself are both critical. The sample retrieving process must not change the composition or condition of the sample. Additionally, when a sample is removed, the sampling site must be repaired to prevent any stress concentrations from forming at the site.
On a turbine rotor, the highest stresses occur on the surface of a bore that runs along the rotor longitudinal axis. Because the metal at the surface of the bore should be in relatively worse shape than the metal in other parts of the shaft, samples taken from the surface of the bore should represent the worst case condition of the rotor as a whole. However, because access to the rotor bore is usually limited, it is difficult to remove a sample from the bore of sufficient size for standard tests, such as a "Charpy" test for example. The limited access to the bore also makes it difficult to repair any damage caused by the sample removal process.
Beyond testing, it is often desirable to remove material from a metal object for the purpose of removing material defects. When a crack forms on a turbine rotor, for example, the crack must be removed before it expands and threatens the mechanical integrity of the rotor. If the crack forms on the outer surface of the turbine rotor, it may be removed by grinding down and polishing the surface of the rotor. However, since the surface of the turbine rotor bore is subject to large stresses, cracks often form on the surface of the bore. Removing a crack that forms on the inner surface of a turbine rotor is much more difficult and expensive than removing one that forms on the outer surface of the rotor and generally requires expensive milling operations.
Various devices have been developed for removing sections from the surfaces of bores and other surfaces on metal objects. Some of these devices remove sections that are large enough to use with standard tests but the removal process damages the object so badly that major repair work is required to prevent stress concentrations from forming at the sampling site. One device, for example, cuts a v-shaped groove from an inner wall of the object. The v-shaped void remaining in the object, however, must itself be removed from the object to prevent stress concentrations from forming at the apex of the v-shaped void.
U.S. Pat. Nos. 4,845,896 and 4,856,233 to Mercaldi disclosed a device that operated to remove a section of material from the inner surface of a pipe. The Mercaldi device included a hemispherically shaped cutter capable of spinning about an axis parallel to the pipe axis and articulating downwardly while continuing to spin. A cutting edge of the spinning cutter cuts into the inner surface of the pipe taking a small dimple-shaped sample. However, the sections taken by the Mercaldi device were too small to use with standard tests, such as the "Charpy" test. Additionally, this device generally removed only a shallow surface section which was not representative of the object as a whole. Further, because the samples were small, the removal process would probably change the properties of the section so that it was no longer representative of the metal object. Also, the Mercaldi hemispherically shaped cutter left a void that required milling to prevent stress concentrations.